1. Field of the Invention
The present invention is generally directed to optical systems, and more particularly to catadioptric optical systems.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of an IC, flat panel display, or other device. This pattern can be transferred onto all or part of the substrate (e.g., a glass plate, a wafer, etc.), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
The patterning device can be used to generate, for example, an IC pattern. The patterning device can additionally or alternatively be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
After patterning the substrate, measurements and inspection are typically performed. The measurement and inspection step typically serves two purposes. First, it is desirable to detect any target areas where the pattern in the developed resist is faulty. If a sufficient number of target areas are faulty, the substrate can be stripped of the patterned resist and re-exposed, hopefully correctly, rather than making the fault permanent by carrying out a process step, e.g., an etch, with a faulty pattern. Second, the measurements may allow errors in the lithographic apparatus, e.g., illumination settings or exposure dose, to be detected and corrected for in subsequent exposures.
However, many errors in the lithographic apparatus cannot easily be detected or quantified from the patterns printed in resist. Detection of a fault does not always lead directly to its cause. Thus, a variety of off-line procedures (i.e., procedures carried out in addition to normal processing of the substrate) for detecting and measuring errors in the lithographic apparatus are known. These may involve replacing the substrate with a measuring device or carrying out exposures of special test patterns, e.g., at a variety of different machine settings. Such off-line techniques take time, often a considerable amount, reducing production time and during which the end products of the apparatus will be of an unknown quality until the measurement results are made available.
In-line measurement and inspection procedures (i.e., procedures carried out during the normal processing of the substrate) are known. For example, scatterometry is an optical metrology technique that can be used for in-line measurements of critical dimension (CD) and overlay. There are two main scatterometry techniques:
(1) Spectroscopic scatterometry measures the properties of scattered light at a fixed angle as a function of wavelength, usually using a broadband light source, such as xenon, deuterium, or halogen based light source such as a xenon arc lamp. The fixed angle can be normally incident or obliquely incident.
(2) Angle-resolved scatterometry measures the properties of scattered light at a fixed wavelength as a function of angle of incidence, usually using a laser as a single wavelength light source.
Using scatterometry the structure giving rise to a reflected spectrum is reconstructed, e.g., using real-time regression or by comparison to a library of patterns derived by simulation. Reconstruction involves minimization of a cost function. Both approaches calculate the scattering of light by periodic structures. The most common technique is Rigorous Coupled-Wave Analysis (RCWA), though light scattering can also be calculated by other techniques, such as Finite Difference Time Domain (FDTD) or Integral Equation techniques.
Known scatterometers, however, have several drawbacks. For example, conventional scatterometers only detect one wavelength at a time. As a result, spectra with more than one wavelength have to be time-multiplexed, which increases the total acquisition time taken to detect and process the spectra.
Given the foregoing, what is needed is an apparatus that can be used for in-line measuring and inspection in a lithographic apparatus.